Currently, fuel cells such as proton exchange membrane (PEM) fuel cells have significant difficulties when starting in cold environments, particularly at or below subzero temperatures. It is widely believed that during PEM fuel cell operation at subzero temperatures water produced from the oxygen reduction reaction (ORR) forms ice in the cathode catalyst layer (CL) that hinders the oxygen transport to the reaction sites, which can cause the PEM fuel cell to eventually shut down. Several technologies attempt to address operating fuel cells in cold environments.
For example, U.S. Pat. No. 6,358,638 B1 discloses an in-situ chemical heating method to produce heat during cold start and hence warm-up a fuel cell stack towards the freezing point. In this method, either a small amount of O2 is injected into the anode to induce O2—H2 combustion in the anode catalyst layer, thereby producing heat. Or a small amount of H2 is injected into the cathode to induce H2—O2 combustion in the cathode catalyst layer for heat production. In both cases, the method is not effective as it also produces water which turns into ice and fills up the catalyst layer so that the fuel cell becomes inoperable. The amount of heat produced by this method is limited by the water storage capacity of the catalyst layer and is rather small due to the small void space in a thin catalyst layer. Additionally, this method incurs degradation of the catalyst layer as the H2—O2 catalytic reaction in the anode catalyst layer will promote carbon corrosion in the cathode catalyst layer, and H2—O2 catalytic reaction in the cathode catalyst layer may result in hot spot formation over the membrane.
U.S. Pat. No. 8,263,278 B2 discloses an oxygen starvation technique to maintain a low cell voltage and hence low-efficiency operation such that there is more internal heat generated to warm up a fuel cell stack. This method of oxygen starvation leads to hydrogen pumping from the anode to cathode compartment, thereby requiring dilution of the cathode exhaust in order to keep the hydrogen concentration below a flammability limit before emitting into the ambient. The oxygen starvation method also requires elaborative control steps and may cause degradation of fuel cell materials.
Therefore, it is desirable to develop a simple, non-degrading method to rapidly start-up a fuel cell from subzero temperatures.
Further, water management in fuel cells operating from low ambient temperatures, e.g. room temperature, before reaching an optimal range, e.g. 60-80° C., has been exceedingly difficult. Bulky humidification systems along with sophisticated controls are employed in order to properly manage water and prevent electrode, gas-diffusion layer and flow channel from flooding by liquid water. A simple approach to this water management problem at low temperatures could be a thermal method in which a cell is quickly heated up from room-temperature to the design point of elevated temperature. The higher cell temperature dramatically promotes water evaporation and removal through vapor phase diffusion, thereby alleviating flooding of fuel cell components and materials.